NCP1028 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1028

High−Voltage Switcher for Medium Power Offline SMPS Featuring Low Standby Power

ON Semiconductor 

NCP1028

Jittering

Frequency jittering is a method used to soften the EMI

signature by spreading the energy in the vicinity of the main

switching component. The NCP1028 offers a "6%

deviation of the nominal switching frequency. The sweep

sawtooth is internally generated and modulates the clock

up and down with a fixed frequency of 300 Hz. Figure 40

shows the relationship between the jitter ramp and the

frequency deviation. It is not possible to externally disable

the jitter.

Jitter ramp

68.9kHz

65kHz

61.1kHz

adjustable

Internal

sawtooth

Figure 40. Modulation Effects on the Clock Signal by the

Jittering Sawtooth

Skip−Cycle

Skip cycle offers an efficient way to reduce the standby

power by skipping unwanted cycles at light loads.

However, the recurrent frequency in skip often enters the

audible range and a high peak current obviously generates

acoustic noise in the transformer. The noise takes its origins

in the resonance of the transformer mechanical structure

which is excited by the skipping pulses. A possible

solution, successfully implemented in the NCP1200 series,

also authorizes skip cycle but only when the power demand

as dropped below a given level. This is what Figure 41

shows, as implemented on the NCP1028.

Nominal peak

current

Skip cycle

current limit

0

Figure 41. Low Peak Current Skip Cycle Guarantees Noise−Free Operation

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